Gel Electrophoresis

Don Lee, Presenter Author

10/10/2018 Added

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Gel Electrophoresis is a common technique scientists use to visualize DNA changes or differences. This video walks you though how this technique works.

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[00:00:13.230]Gel electrophoresis is a very common
[00:00:16.500]and routine technique that molecular geneticists will use
[00:00:21.570]to actually look at copies of DNA.
[00:00:24.660]So, we're gonna run through this USDA-funded animation
[00:00:28.920]developed by the University of Nebraska,
[00:00:31.260]and get an idea of how electrophoresis works.
[00:00:34.770]Gel electrophoresis, as the name implies,
[00:00:37.280]we're gonna use electricity to move DNA,
[00:00:40.250]and we have to make a gel matrix
[00:00:43.080]that's really a lot like making a Jell-O.
[00:00:46.802]Okay, so, we can kind of get an idea, yeah,
[00:00:51.060]it's this gel, it's this clear component, that's in a mold.
[00:00:58.560]Once it's made you can carefully load your samples in there.
[00:01:01.760]So we're gonna let the animation
[00:01:03.790]kind of visualize this process.
[00:01:06.800]So when scientists look at a gel,
[00:01:08.640]they always look at it from the top.
[00:01:10.400]But we're gonna also look at it from the side,
[00:01:12.520]so we can see how the gel matrix
[00:01:15.860]actually helps us separate DNA molecules
[00:01:20.060]based on how they move through the gel.
[00:01:22.330]All right, so, let's see here.
[00:01:24.010]We've got some choices.
[00:01:26.390]We've got a sample that's just got one length of DNA,
[00:01:31.210]let's try Sample B, it's got two different lengths of DNA.
[00:01:34.240]So, these would be samples of DNA that we've got copies of
[00:01:41.570]using the polymerase chain reaction or PCR technique.
[00:01:46.220]This particular sample may have also been treated
[00:01:49.190]with a restriction enzyme that will cut the DNA
[00:01:52.870]if a specific sequence is available.
[00:01:55.340]The key is, we've got a sample
[00:01:57.010]that's got two different lengths of DNA,
[00:01:59.220]and so we wanna understand how the gel works
[00:02:01.710]to allow us to visualize
[00:02:03.370]that that's what we've got in our sample.
[00:02:05.520]So, we carefully pipette out our sample,
[00:02:08.950]put it in the little well that was left behind
[00:02:14.040]when we removed a cone, okay, in our electrophoresis.
[00:02:18.190]So now we've got all of our DNA loaded
[00:02:20.600]in this well and we want the DNA to move.
[00:02:23.450]DNA has a negative charge and so if we can put it up
[00:02:27.950]to the current, it'll move towards the positive pole.
[00:02:32.850]All right, so the animator helped us visualize
[00:02:35.750]this gel matrix.
[00:02:37.170]It's mostly water.
[00:02:38.320]When you make Jell-O, it's mostly water,
[00:02:41.130]a little bit of powder and sugar.
[00:02:42.880]And so these dark gray would be the gel,
[00:02:46.987]the agarose gel particles.
[00:02:49.710]But most of it is the matrix, the space in between,
[00:02:54.441]and since it's made with water,
[00:02:56.020]these are full of water.
[00:02:57.080]And then we have water in the tank to conduct electricity.
[00:03:00.120]So now let's see what happens as we let the DNA
[00:03:04.800]move in response to the current.
[00:03:06.700]Okay, as the DNA moves, what's happening here?
[00:03:10.340]Let's see, let's go back and watch that again.
[00:03:13.090]So DNA moves, the bigger fragments aren't moving
[00:03:16.580]as fast as the smaller fragments, okay?
[00:03:19.500]The bigger fragments just have a harder time
[00:03:22.450]moving around the different agarose particles.
[00:03:26.480]The smaller fragments can move faster.
[00:03:28.650]And since we have lots of copies,
[00:03:30.930]when we look at it from the top,
[00:03:32.450]we look through the gel and see lots
[00:03:34.720]of copies that are smaller.
[00:03:36.660]They moved faster through the gel
[00:03:39.630]than the larger copies.
[00:03:42.540]Lots of copies of the larger fragment
[00:03:45.120]form a band that hasn't moved as far from the well.
[00:03:49.670]All right, so that's how electrophoresis works.
[00:03:52.740]What if we had a sample that was just one size fragments?
[00:03:56.180]Let's run sample A and see if your thinking
[00:03:59.727]goes along with what we-
[00:04:02.130]or if you would predict the right thing.
[00:04:04.500]We pull out our sample, all bigger fragments.
[00:04:09.020]Load it in our gel.
[00:04:16.070]Hook up the power.
[00:04:20.610]And again, the bigger fragments move through the matrix,
[00:04:25.000]but the bigger fragments don't move as fast
[00:04:27.490]as smaller fragments.
[00:04:28.640]But since they're all the same size,
[00:04:30.470]they move at about the same rate.
[00:04:32.230]If we have lots and lots of these copies,
[00:04:34.130]when we look at the top of the gel,
[00:04:35.980]we'll just see one band.
[00:04:37.090]Okay, that's how electrophoresis works.
[00:04:39.860]That's how scientists can look at the copies
[00:04:43.670]of the DNA that they've made with PCR
[00:04:47.450]and by comparing different samples
[00:04:51.910]you can get an idea of the similarities
[00:04:56.850]at the DNA sequence level
[00:05:00.900]between your different samples.
[00:05:02.890]Okay, so it's a pretty powerful technique
[00:05:05.050]that allows scientists to look at DNA.

Gel Electrophoresis

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